National security adviser Jake Sullivan said Sunday that preventing Iran from obtaining a nuclear weapon remains a “paramount priority” for the United States, emphasizing that diplomacy “is the best way to achieve that.”

Sullivan weighed in on the issue during appearances on the Sunday morning news shows one day after the election of Iran’s new president, Ebrahim Raisi, who was previously the country’s ultraconservative judiciary chief.

“Well, I think what we need to do in the United States is keep our eye on the ball,” Sullivan said when asked about Iran on ABC News’s “This Week.” “And that is — our paramount priority right now is to prevent Iran from getting a nuclear weapon. We believe that diplomacy is the best way to achieve that, rather than military conflict. And so, we’re going to negotiate in a clear-eyed, firm way with the Iranians to see if we can arrive at an outcome that puts their nuclear program in a box.”

Sullivan added that the United States believes the decision on whether to revive the 2015 nuclear deal lies not with Raisi but with Iran’s 82-year-old supreme leader, Ayatollah Ali Khamenei.

“He was the same person before this election as he is after the election, so ultimately, it lies with him,” Sullivan said.

Raisi’s win is not expected to derail negotiations that are underway in Vienna between Tehran and world powers on the possible revival of the nuclear deal. Khamenei has allowed Iran to reopen the dialogue and appears ready to keep it going in efforts to get international sanctions on Iran lifted. But the longer-term effect on Iran’s relationships with Europe and the United States was far less clear.

A day after an election victory by Iran's Ebrahim Raisi, Israeli Minister Naftali Bennett urged “world powers to wake up before returning to” a nuclear deal. (Reuters)

On the issue of North Korea’s nuclear program, Sullivan said “time will tell” whether another round of multilateral negotiations is possible.

“We are awaiting a clear signal from Pyongyang as to whether they are prepared to sit down at the table to begin working in that direction,” he said. Sullivan added that North Korean leader Kim Jong Un’s recent comments are “an interesting signal” but that the administration continues to wait for a “more direct communication to us about a potential path forward.”

According to the Korean Central News Agency, during a ruling-party meeting last week, Kim “stressed the need to get prepared for both dialogue and confrontation” with the United States — “especially to get fully prepared for confrontation in order to protect the dignity of our state,” the Associated Press reported.

Sullivan on Sunday also gave an assessment of President Biden’s first foreign trip, explaining how the administration views relations with Russia and aiding Ukraine.

During an interview on CNN’s “State of the Union,” Sullivan said the United States is continuing to prepare another package of sanctions against Russian targets over the poisoning of Russian opposition leader Alexei Navalny, responding to Senate criticism that the White House had missed a deadline to issue the sanctions.

“It will come as soon as we have developed the packages to ensure that we are getting the right targets, and when we do that, we will impose further sanctions with respect to chemical weapons,” Sullivan said.

Sen. Robert Menendez (D-N.J.), chairman of the Senate Foreign Relations Committee, recently criticized the Biden administration for missing a congressionally mandated deadline of early June to impose the sanctions, saying Russian President Vladimir Putin has “shown no remorse” over the poisoning.

Sullivan noted that the United States has already issued one round of sanctions over the poisoning in concert with European allies.

In the wake of Biden’s meeting with Putin in Geneva last week, Sullivan said the United States will see “through actions, not through words or commitments or body language, whether in fact we’re on a better track with U.S.-Russia policy and the U.S.-Russia relationship” over the next six to 12 months.

Biden is “not taking anything for granted from this meeting,” Sullivan said.

Asked on “Fox News Sunday” to respond to Republican criticism of Biden’s meeting with Putin, Sullivan argued that there is no comparison between how Biden represented the United States on the world stage and how Donald Trump did during the 2018 Helsinki summit.

Sullivan was also asked to explain why the United States had frozen its aid package to Ukraine, another recent criticism from conservatives. Sullivan noted that there has “been confusion in the reporting,” especially when it comes to what has already been allocated.

He said the United States has given Ukraine $275 million in aid and had promised an additional $100 million if Russia continued to pose an imminent threat by building up troops along the border with Ukraine. Given that the threat had subsided, the United States froze that extra $100 million. But Sullivan cautioned that the United States stands ready to help Ukraine if Russia takes aggressive actions.

“The idea that we have withheld any security assistance from Ukraine is simply nonsense,” he said.

Sullivan reiterated that Biden was terse with Putin behind closed doors, making it clear that there would be consequences if Russia continues to take actions detrimental to the United States.

“Privately in the room, President Biden communicated to President Putin that there would be costs and consequences if harmful activities against the United States continued,” Sullivan told host Chris Wallace. “Publicly, in his press conference, he not only spoke out about that quite directly, mincing no words, but he also spoke about American values — something the last president never talked about.”

Kareem Fahim contributed to this report.